Methods of synchronizing alternating current (AC) synchronous electrical power generators to an AC electrical power grid are well known in the art. When an electrical generator is started or re-started, it is initially isolated from the electrical grid. As power is applied to the prime mover, the generator takes time to reach its normal operating speed. With no load or minimal auxiliary load on the generator, the output voltage and frequency of the generator will eventually reach that of the electrical grid to which it will be connected. However, the generator cannot be connected to the grid until the output voltage or current of the generator is at nearly the precise point on its sinusoidal waveform as that of the voltage or current of the electrical grid. When the two waveforms are nearly precisely “in synchronism,” or “in phase” with each other, a circuit breaker or circuit switcher that ties the generator to the grid can be closed. The intent of this process is to minimize energy flow across the breaker or switcher as it is closed and to lessen any shock to the generator or other connected rotating machinery. Differences in phase angle between the generator and the grid when a breaker is closed will result in large amount of energy to flow across the contacts of the breaker as it is closing. Thus, this process of synchronization is very delicate. If done improperly, i.e., closing the output breaker when the generator and power system are too far out of voltage phase alignment, it can result in severe and permanent damage to the generator, prime mover, or circuit breaker.
In the typical application of synchronizing a generator to the electrical system during a generator start-up, a synchronizing relay is commonly used to determine when it is permissible to close the circuit breaker or switcher. Modern synchronizing relays are solid-state or microprocessor-based devices which electronically compare the output waveform of the generator to that of the electrical system. These devices are designed only to synchronize an isolated generator from an electrical system by comparing the waveforms on either side of an open circuit breaker. A typical synchronizing relay cannot detect an abnormality on the grid beyond the generator bus, although such abnormalities may cause an asynchronous condition. For example, in many parts of the U.S., generation facilities are located in remote rural areas so as to be closer to the fuel source and further from residential areas. These generators are often tied to the grid with single transmission lines. Typically, if protective relays detect a fault on such a transmission line, a circuit breaker will isolate the line temporarily, but it will automatically reclose after a few seconds (since most transmission line faults are transient in nature). During the time that the circuit breaker is open in this case, the generator is temporarily isolated, or “islanded.” The generator may continue operating for those few seconds that the generator is islanded, depending on the load in the area, thus roughly maintaining the system voltage and frequency at the generator bus. However, since the generator is now electrically isolated, it is very likely that the phase angle will shift during this short period of time, relative to the rest of the electric grid. Therefore, if the transmission line circuit breaker recloses without synchronism check supervision, the generator will be closed into the grid in an asynchronous condition, which could potentially cause significant damage to the generator, the prime mover, or the circuit breaker.
Many protection schemes exist to detect and isolate electrical system faults. The challenge for protection engineers is balancing sensitivity with selectivity, i.e., a protection system must be able to detect faults on the devices being protected, but they should not operate for faults beyond their zone of protection (otherwise generation and transmission systems would be tripping repeatedly). Generators are usually equipped with a host of protective relays that will isolate the generator under certain system conditions, including islanding from the grid caused by a remote circuit breaker trip. For example, under/over voltage and under/over frequency relays will eventually detect an islanded condition, since the speed of the generator will change as the generator struggles to match the remaining load. But these relays are relatively insensitive (although very selective), especially if the load in the area surrounding the generator is reasonably matched to the generator output. Therefore, these relays may not respond before the reclose cycle of the transmission line circuit breaker reconnects the generator with the system in an asynchronous condition. A relatively new type of relay, called a vector shift or vector surge relay, has been developed to detect and respond to the generator islanding situation. These relays monitor the sinusoidal waveform of the voltage produced by the generator, and are set to detect a sudden phase shift or rate of change in frequency of the sinusoidal signal, which may indicate a remote asynchronous or islanding condition. The vector shift relay is designed to only compare the relative shift in phase from one cycle to the next. Unfortunately, this sudden phase shift will also occur during many other types of faults on the generator or the grid, so this solution is often considered overly sensitive and not selective enough, thus causing many false generator trips. Another solution to the problem of generator islanding caused by remote circuit breaker trips is to place traditional synchronizing relays at each of the remote circuit breakers which, if tripped and reclosed, could cause an asynchronous condition. In a typical electric grid, however, this could require dozens of synchronism check relay installations, along with required ancillary equipment, which would be cost prohibitive. In addition, the setting and installation of a synchronism check relay could be required in a section of the power grid not owned or controlled by the entity that owns or controls the generator, thus resulting in a potential conflict of interest.
A method for protecting an electrical generator which avoids the shortcomings attendant with the prior art devices and practices utilized heretofore is the subject matter of the present application.